Perceived Visual Speed while Walking: More than Subtraction

Perceived visual speed has been reported to be reduced during walking compared to standing still. This so-called ‘subtraction effect’ has been attributed to an automatic subtraction of part of the walking speed from the visual speed [1]. In this study, we investigated how general this subtraction effect is, by varying visual speed, walking speed and the order of the intervals in which observers walked or stood still. Five observers judged the visual speed of a simulated ground plane that was presented on a HMD in a 2IFC task. In one interval, they judged the visual speed while walking in place on a treadmill (0.6, 1.0, or 1.4 m/s), and they did the same while standing still in the other interval. Simulated visual standard speed, presented during walking, was 1.0, 2.0, or 3.0 m/s. All observers compared the three visual standard speeds during the three walking speeds against a range of visual test speeds during standing still and indicated in which of the two intervals the visual speed appeared to be higher. For three of the observers the order of the intervals was standing—walking, while it was reversed for the other two observers. From the speed judgments, the PSE’s in the nine conditions were estimated by fitting psychometric functions. Surprisingly, the PSE’s were hardly affected by walking speed. Visual standard speed strongly affected visual speed judgments for the observers who first stood still and then walked. The lowest standard speed was reported to be perceived as slower during walking than during standing still, while the opposite was true for the highest standard speed. When observers first walked and then stood still, this effect did not occur. Taken together, the results question the generality of the subtraction effect and raise doubts regarding the hypothesized functional role of this effect